Purification of in-polymer GlcUA-5-epimerase will be continued, using 5-3H-labeled, partially N-sulfated (GlcNAc - GlcUA)n as assay substrate and release of 3H as the assay reaction. Enzyme source will be mammalian liver or lung soluble fraction. Heparin synthesized from specifically 3H-labeled GlcUA precursors will be converted chemically to the component uronic acids and the latter will be degraded carbon by carbon to locate the 3H. Shift of label will be used to help determine the mechanisms of the in-polymer 5-epimerization of GlcUA which occurs during heparin biosynthesis. The role of ester intermediates in the uronosyl C-5 epimerization of D-glucoronic acid will be investigated by examination of the role of experimental conditions or the chemical and enzymic incorporation of label (3H or 2H) into heparin and its analogues.